Precision machine vision inspection systems can be used to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a user interface, a lighting system, a camera and optical system, and a precision stage that is movable in multiple directions to allow an operator to position the camera to image various features of a workpiece. The user interface, among other things, generally includes various video tools that are positionable on an inspection image. In this way a user of the machine vision inspection system can position and operate the video tools to perform image processing operations that are useful for various control and inspection operations, while having little or no knowledge of image processing. One exemplary prior art system having such features, of a type that can be characterized as a general-purpose “off-line” precision vision system, is the commercially available QUICK VISION™ series of vision inspection machines and QVPAK™ software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION™ series of vision inspection machines, and the QVPAK™ software, including the user interface and various video tools are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine Users Guide, published January 2003 and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is incorporated herein by reference in its entirety. This product, as exemplified, for example, by the QV-302 Pro model, uses a microscope-type optical system to provide images of a workpiece at various magnifications, and includes all of the features outlined above.
Such general-purpose “off-line” precision vision systems are characterized by their versatility, and they provide the ability for a user or an automatic program to rapidly change their configuration and imaging parameters in order to perform a wide variety of inspection tasks on various types of objects or inspection workpieces, or various aspects of a single workpiece.
General purpose precision machine vision inspection systems, such as the QUICK VISION™ system, are also generally programmable and operable to provide automated video inspection. It is generally desirable that such systems include features and tools that simplify the programming and operation of such systems, such that operation and programming can be performed reliably by “non-expert” operators.
Automated video inspection metrology instruments generally have a programming capability that allows an automatic inspection event sequence to be defined by the user for each particular workpiece configuration. The programming capability also typically provides the ability to store and/or output the results of the various inspection operations. Such programming can be implemented either in a deliberate manner, such as text-based programming, for example, or through a recording mode that progressively “learns” the inspection event sequence by storing a sequence of machine control instructions corresponding to a sequence of inspection operations performed by a user, or through a combination of both methods. Such a recording mode is often referred to as “learn mode,” “training mode,” or “teach mode.”
In these techniques, the machine control instructions are generally stored as a part program that is specific to the particular workpiece configuration. The ability to create part programs with instructions that automatically perform a predetermined sequence of inspection operations during a “run mode” of operation provides several benefits, including enhanced inspection repeatability, as well as the ability to automatically execute the same part program on a plurality of compatible machine vision inspection systems and/or at a plurality of times.
In machine vision systems, “occlusion” type problems sometimes arise, that is, situations in which a foreground object interferes with the viewing or inspection of a background object. Occlusion problems have generally not been addressed by general purpose machine vision systems for inspection and measurement of workpieces. Previously, there have been no readily programmable alternatives. In general, the user had to carefully size and place tools using human judgment to avoid the occluding object and/or shadow. In such cases, when inspecting images having foreground and background features, such as edge features, in close proximity in the feature to be inspected, the slightest variation in construction between various workpieces, or lighting and shadows, will cause the carefully positioned and trained tools to fail or provide erroneous results.
Alternatively, various custom-designed region or boundary “growing” and “connection” processes have been used to “reconstruct” a background object feature. However, such methods are time consuming to implement and require considerable knowledge. Furthermore, such methods are actually creating artificial features without actually increasing the “real” information available in the image. Therefore, such methods introduce risk that a particular inspection operation may return results based primarily on the artificial features rather than the original image portions that are known to be real and valid.
As a further alternative, various custom-designed image filtering processes have been designed by specialists to remove the unwanted occluded image features. However, such filtering processes also filter the desired image feature, which alters its characteristics to some extent. In many cases this is undesirable, especially for various precision metrology operations used to inspect a workpiece. For example, certain methods for removing unwanted image features apply morphology filters, such as closing and opening, to “delete” the unwanted image features, and then measure the desired features with standard edge detection tools on the filtered image. Closing and opening filters are effective for removing the unwanted image features, however, the closing and opening filters also modify the location of the edges. Therefore, the edge points that are detected lack the desired accuracy. All of the previously described methods provide a poor basis for quality control and inspection operations, particularly in a general purpose machine vision inspection system intended to provide reliable operation and a relatively simple programming environment for relatively unskilled operators.
The present invention is directed to a system and method that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to a method of measuring occluded features.